1. Field of the Invention
This invention relates generally to surface planarization processes for the fabrication of magnetic heads and other devices (such as semiconductor devices), and more particularly to processes that eliminate or reduce surface “steps” formed between different materials by selectively bonding polymer materials to one of the materials.
2. Description of the Related Art
A write head is typically combined with a magnetoresistive (MR) or giant magnetoresistive (GMR) read head to form a merged head, certain elements of which are exposed at an air bearing surface (ABS). The write head is made of first and second pole pieces having first and second pole tips, respectively, which terminate at the ABS. The first and second pole pieces are connected at the back gap by a yoke, whereas the first and second pole tips are separated by a non-magnetic gap layer. An insulation stack, which comprises a plurality of insulation layers, is sandwiched between the first and second pole pieces, and a coil layer is embedded in this insulation stack. A processing circuit is connected to the coil layer for conducting write current through the coil layer which, in turn, induces write fields in the first and second pole pieces. Thus, write fields of the first and second pole tips at the ABS fringe across the gap layer. In a magnetic disk drive, a magnetic disk is rotated adjacent to, and a short distance (fly height) from, the ABS so that the write fields magnetize the disk along circular tracks. The written circular tracks then contain information in the form of magnetized segments with fields detectable by the read head.
One or more merged heads may be employed in a magnetic disk drive for reading and writing information on circular tracks of a rotating disk. A merged head is mounted on a slider that is carried on a suspension. The suspension is mounted to an actuator which rotates the magnetic head to locations corresponding to desired tracks. As the disk rotates, an air layer (an “air bearing”) is generated between the rotating disk and an air bearing surface (ABS) of the slider. A force of the air bearing against the air bearing surface is opposed by an opposite loading force of the suspension, causing the magnetic head to be suspended a slight distance (flying height) from the surface of the disk.
Improved methods for making magnetic heads have become increasingly important for proper head fabrication and performance. Magnetic head assemblies are typically made of multiple thin film layers which are patterned to form various shaped layers in the head. Some of the layers are electroplated, while other layers are sputter deposited on a wafer substrate. Photolithography processes are typically utilized to create very small track widths for the magnetic heads, resulting in increased storage capacity in magnetic disks.
During a photolithography process, a mask image or pattern which defines the various components is focused onto a photosensitive layer using ultraviolet light. The image is focused onto the surface using an optical device of a photolithography tool, and is imprinted into the photosensitive layer. To build increasingly smaller structures, increasingly fine images must be focused onto the surface of the photosensitive layer (i.e., the optical resolution must increase). As optical resolution is increased, the depth of focus of the mask image is correspondingly narrowed due to the narrow range in depth of focus imposed by the high numerical aperture lenses in the photolithography tool. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable (and thus the smallest structures obtainable) using the photolithography tool. The extreme topography (i.e., the “hills” and “valleys” along the surfaces) exaggerates the effects of decreasing depth of focus. Thus, in order to properly focus the mask image defining sub-micron geometries onto the photosensitive layer, a precisely flat surface is desired. A precisely flat (i.e. fully planarized) surface will allow for extremely small depths of focus and, in turn, allow the definition and subsequent fabrication of extremely small structures.
Typically, a chemical mechanical polishing (CMP) is utilized as the means of reducing the topography in order to achieve adequate critical dimension (CD) control. CMP involves removing at least a portion of a sacrificial layer of dielectric material using mechanical contact between the wafer and a moving polishing pad saturated with slurry. Polishing flattens out most height differences, since high areas of topography (“hills”) are removed faster than areas of low topography (“valleys”). Such polishing is the only technique with the capability of smoothing out topography over millimeter scale planarization distances.
When two or more different materials form the top surface, however, surface “steps” may remain between the materials even after CMP. It has been observed that, after the CMP process in magnetic head manufacturing, surface steps in the range of 10–300 nm remain between the materials. One surface material may be a metal and the other material may be a dielectric; these materials have very different CMP removal rates. This non-ideal situation adversely affects subsequent processing steps (e.g. photolithography steps). A non-planar surface changes the thickness distribution of the subsequently deposited materials and increases the chance that surface scattering will occur.
FIG. 1 is the first in a series of illustrations of FIGS. 1–3 which describes in more detail the problem of conventionally forming a planar surface using a CMP process. An initial structure 100 includes a substrate 102 having a plurality of first material structures 104. In this example, first material structures 104 include structures 106,108, and 110. Since structures 106,108, and 110 cover only portions of substrate 102, recesses (such as recesses 112 and 114) are formed between structures 106, 108, and 110 and the exposed portions of substrate 102. In the fabrication of magnetic heads, substrate 102 is typically a metal or a magnetic material (such as a pole piece layer of a magnetic head) or alternatively a non-magnetic material or an insulator. First deposited material 104 is typically a metal or a magnetic material (such as a pedestal of the pole piece).
It is desired to fill in the recesses with a material (e-g. an insulator) in an attempt to form a top planar surface with the tops of structures 106,108, and 110. This is done so that another material (e.g. a metal or a magnetic material) can be deposited over the surface and contact can be made with it and the tops of the first material structures 104. To illustrate, it is shown in FIG. 2 that a second material 202 is deposited over and around these first material structures 104. Second material 202 may be an insulator, such as alumina (Al2O3). In FIG. 3, it is shown that a chemical mechanical polishing (CMP) is performed over the structure to remove top surface portions of second material 202 such that the tops of first material structures 104 are exposed and a top surface is formed from the tops of first and second materials 104 and 202.
The top surface formed from the tops of first and second materials 104 and 202 is somewhat flat. Since first and second materials 104 and 202 have different CMP removal rates, however, small surface “steps” between these materials remain along the top surface even after the CMP (i.e., the resulting top surface is not entirely coplanar). The surface steps between first and second materials 104 and 202 (such as a step 310) may be, for example, in the range of about 10–300 nm. Again, this non-ideal situation adversely affects subsequent processing steps (e-g. photolithography steps) during the formation of the magnetic head.
Accordingly, what are needed are improved surface planarization processes for the fabrication of magnetic heads or other devices such as semiconductor devices.